Mark McDonough pointed out the persistent nature of Tulipa tarda in his New
England garden. It is the same here (until the voles find it).
This isn't very good tulip country either, owing to the wet winter/spring
weather, but some small species tulips seem relatively permanent and
increase: Tt. urumiensis, hageri, humilis 'Lilliput' (I suspect that clone
is not T. humilis, however), linifolia, eichleri, saxatilis, sylvestris,
and clusiana. Most of these do best in a rock garden situation, but T.
sylvestris likes a moister, more retentive soil. T. clusiana is well known
for its adaptability to warm Mediterranean climates; I gave my mother some
that spread nicely among really aggressive tree roots.
A delightful tulip species that is not seen much in North American gardens
is T. sprengeri, which flowers very late, usually in June here. It is a
medium-sized bright red one that is apparently difficult to propagate in
the usual way; I believe it makes very tiny offsets, perhaps on stolons.
Seed is readily available, however, and I've raised a few that way.
Jane McGary
Northwestern Oregon, USA